In the method for obtaining an image by an electrophotographic printing method and an electrostatic printing method, an electrostatic image formed on a photosensitive member is developed with toner charged in advance by friction and then fixed. As the fixing technique, there are a heat roller technique in which the toner image is fixed by using a pressurized and heated roller and a non-contact fixing technique in which the toner image obtained by development is fixed using an electric oven or flash beam light. In order to pass through these processes without any problem, toner is firstly required to be able to maintain a stable charged amount, and next, to exhibit a favorable fixing property on the paper. Furthermore, as the device has a fixing part as a heating member and there is a temperature increase inside the device, toner is required not to cause a blocking. It is also required that, even during continuous printing, no contamination of the device or fog on a printed surface is not shown, namely, durability of toner is required.
Moreover, with regard to the heat roller technique, having low temperature in a fixing part is under progress from the viewpoint of energy saving, and the toner is strongly required to have a characteristic of being fixed on paper at relatively low temperature, i.e., to have a low temperature fixing property. In addition, since having a device in more compact form is now under progress and a roller not coated with a release agent is used, toner is strongly required to have a peeling property from a heat roller, that is, the demanded for offset resistance has been increasing.
As a binder resin for toner, a polyester resin, which is excellent in toughness and fixing property at a low temperature and exhibits a favorable performance balance, receives particular attention in recent years.
From the viewpoint of energy saving or the like, it is recently desired to have a binder resin which can provide toner having good fixing property at a low temperature. As for the polyester resin, due to the reason of having also the hot offset resistance at a high temperature, having a design such that a branched or crosslinked structure is included in the resin by using a tri- or higher hydric carboxylic acid component or alcohol component is widely carried out in general. However, there is a problem that, when a large amount of a tri- or higher hydric monomer is copolymerized, it is difficult to control the polycondensation reaction.
With regard to the problem described above, disclosed in Patent Literature 1, for example, is a method of suppressing a rapid increase in viscosity in reaction system even for a case of copolymerizing a large amount of tri- or higher hydric carboxylic acid component by adjusting the vacuum degree within a predetermined range during polycondensation reaction. Furthermore, in Patent Literature 2, for example, polyester for toner containing aliphatic diol in a specific amount or more and tri- or higher hydric carboxylic acid in which metal elements are not more than a specific amount is disclosed. According to Patent Literature 2, a binder for toner which does not substantially contain a polymerization catalyst and is excellent in transparency and electrostatic property is disclosed.
However, because dibutyl tin oxide is used as a polymerization catalyst in Patent Literature 1, there is concern regarding the safety. Furthermore, according to Patent Literature 1, a desired resin cannot be obtained by gellation and, in a case in which a desired resin is obtained, a long period of time is required for polycondensation reaction, or the like so that there is still a room for improvement in terms of production conditions. Incidentally, in Patent Literature 2, aromatic diol is not used as an alcohol component while only an example of using ethylene glycol is given. As such, there is concern regarding the problems associated with toner characteristics like blocking resistance and electrostatic characteristic. Also of concern are, in such molecular structure, easy exhibition of a crystal structure in part of the structure and a negative influence exhibited on the toner characteristics.